A known class of optical sensors for various applications includes the use of intrinsic fiber optic sensors. An intrinsic fiber optic sensor is a sensor that uses an optical fiber as sensing element. In many applications the intrinsic fiber optic sensor is lit (e.g. by a laser), and a change in the fiber due to an external cause (dependent on the type of sensor) results in a measurable change in the optical signal transmitted by or received from the sensor. A typical example of an intrinsic fiber optic sensor is a Fiber Bragg Grating (FBG).
An FBG comprises a fiber having a core including therein, over a certain distance, a periodic variation of the refractive index. This periodic variation forms a wavelength-specific dielectric mirror, wherein light in a specific (narrow) range around and including a certain wavelength is reflected. The wavelength reflected is determined by the periodicity of the refractive index of the core. The FBG is based on the principle that a difference in strain of the optical fiber causes the geometric periodicity of the variations to change. This results in a change of the reflected wavelength, such change can be detected (e.g. by spectral analysis, or using an interferometer).
Fiber Bragg Gratings are applied in many applications for measuring a wide range of parameters and characteristics, including static and dynamic parameters. For example, an FBG may be applied in a pressure sensor by converting an exerted exterior pressure into a variable force applied to the fiber including the FBG. Another application, usable for example to perform geological survey, is the application of an FBG in an accelerometer e.g. for measuring soil vibrations. Such sensors are advantageously applied for example in the oil and gas industry, e.g. to analyze the structure of the soil and to identify cavities or porous rock formations.
For many of these applications, it is required that the size of the optical sensor device is small, for example to enable implementation in a cable that may be useable for exploration and geological survey at land, at sea or in a borehole, or to measure vibrations in a building, a bridge or other construction. However, achieving the desired size reduction often comes at the cost of sensitivity of the sensor, and this trade-off is usually not desired. Moreover, sensitivity and size are not the only characteristics that are of interest. In particular for dynamic sensors, a broad dynamic operational frequency range is desired. An upper limit of this operational frequency range is determined by the resonance frequency of the sensor device. Even for static sensors, fast and accurate response to variations is often required. Matching all these requirements in an optical fiber design is complex because they are related to each other, and meeting one requirement often deteriorates the design for another requirement. Hence, additional measures are required to further enhance the sensitivity of such an optical sensor element, in view of the constraints given.